Field of the Invention
This invention relates to a process for preparing a foreproduct useful in the production of an optical self-focusing lightconductor in which the refractive index of the lightconductor material decreases with increasing distance from the conductor axis such that, when plotted against the conductor diameter, the refractive index curve is parabolic and the apex of the parabola is at the center of the conductor diameter, and in which a plurality of layers of doped silica are deposited successively on a cylindrical support, especially one made of glassy material, the said layers being formed at high temperature from a mixture of a vaporous silicon compound and a vaporous doping agent.